In the US, Toxoplasma gondii is the second leading cause of food borne mortality and more than 60 million people are persistently infected with this organism and this infection is a concern in many patients with primary or acquired defects in T cell mediated immunity. Due to the ease of transmission and its ability to cause severe disease, T. gondii is classified as a Class B Biodefense pathogen. The ability to balance the development of protective and pathological immunity is key in determining the outcome of this infection. This proposal studies the role of the cytokine IL-27 in controllinga unique population of regulatory T cells to limit inflammation and is broadly applicable to understanding how this cytokine may be useful as a therapy.